Electromagnetic waves are radiated from cables of electric appliances and electronic appliances, and ambient electromagnetic waves intrude into cables, so that noises enter signals. To prevent the radiation and intrusion of electromagnetic waves to and from cables, cables are conventionally shielded with metal nets or foils. For example, JP 11-185542 A discloses a cable having a thin magnetic shield film, which comprises an insulating tape wound around pluralities of signal-transmitting cables, a laminate tape of a high-conductivity metal foil (copper foil, etc.) and a thin film of a high-permeability material (permalloy, etc.) wound around the insulating tape, and an insulator wound around the laminate tape. However, it fails to achieve complete shielding for high-frequency noises. Accordingly, it is proposed to prevent the radiation and intrusion of electromagnetic waves by absorption instead of shielding.
JP 2005-259385 A discloses a communications cable comprising pluralities of first pair-core cables each comprising two metal core cables electrically insulated with a rubber sheath, a first sheath covering the first pair-core cables, pluralities of second pair-core cables each comprising two metal core cables electrically insulated with a rubber sheath and arranged outside the first sheath, a second sheath covering the second pair-core cables, a metal net covering the second sheath, and an insulator layer covering the metal net. Each of the first and second sheaths has a two-layer structure comprising a magnetic layer and a conductor layer. The magnetic layer is formed by, for example, a sheet comprising fine amorphous alloy particles bonded by a binder, and the conductor layer is formed by, for example, a sheet comprising fine silver particles bonded by a binder. However, because the sheath having a two-layer structure comprising a magnetic layer and a conductor layer is used in this communications cable, the cable is inevitably thick and expensive.
JP 2009-71266 A discloses a communications cable having a structure comprising a communications core cable or power core cable covered with an insulating layer, an electromagnetic-wave-absorbing/shielding film comprising an electromagnetic-wave-shielding layer and an electromagnetic-wave-absorbing layer being wound around the cable in the insulating layer. As the electromagnetic-wave-shielding layer, an electromagnetic-wave-shielding film having a thickness of 17-70 μm, which is constituted by a polymer film laminated with an aluminum or copper foil or vapor-deposited with Al or Cu, is exemplified. As the electromagnetic-wave-absorbing layer, an electromagnetic-wave-absorbing film having a thickness of 10-100 μm, which is coated with a paint containing metal flake and/or electromagnetic-wave-absorbing Fe—Si alloy powder, Fe—Si—Cr alloy powder, amorphous metal powder, etc., is exemplified. However, because the electromagnetic-wave-absorbing/shielding film has a two-layer structure comprising the electromagnetic-wave-shielding layer and the electromagnetic-wave-absorbing layer both relatively thick, the communications cable is inevitably thick and expensive.
In place of electromagnetic-wave-absorbing layers entirely covering cables as described above, filters covering part of cables for absorbing electromagnetic waves generated from cables and preventing external noises from intruding into cables are also used. Most of conventional electromagnetic-wave-absorbing filters are formed by sintered ferrite or resin/rubber sheets containing sintered ferrite powder. However, such electromagnetic-wave-absorbing filters are disadvantageously bulky, because of their large thickness.